While the invention is subject to a wide range of applications, it is especially suited for use in a pulp bleaching system and will be particularly described in that connection. In making paper, pulp (primarily cellulose fibers and lignin) is processed to separate the cellulose fibers from the lignin which are the primary constituent in the paper. The lignin, a binder for the fibers, is generally of a darker color. In order to produce white paper, it is necessary to remove the lignin from the pulp. To accomplish this, an oxidizing agent, such as chlorine, is added to a pulp slurry. The lignin is oxidized and becomes soluble for easy removal from the pulp.
In the past, a standard practice was to mix a pulp slurry with chlorine in a chlorinator and deliver the resulting mixture to a chlorination tower for allowing the reaction to be completed. The chlorinator serves to both mix the chlorine with the pulp slurry and to allow the reaction to begin before the mixture enters the chlorination tower. This step is important because wet chlorine gas from the mixture might otherwise rise directly up the tower in a phenomenon called channeling. Wet chlorine gas, being very corrosive, requires equipment of a special alloy and careful use to avoid serious maintenance problems of the equipment. Further, when channeling occurs in a chlorination tower, the concentration of chlorine in the pulp slurry decreases and causes a longer time to complete the reaction. Also the chlorine bubbling through the tower carries pulp and uneven bleaching occurs. Further, the loss of this chlorine causes a greater expense in operating the apparatus.
The early chlorinators consisted of static mixers. These are essentially pipes with internal baffles to create turbulence between the chlorine and the pulp slurry and thereby provide mixing. This type of mixer has several disadvantages. First, the gas, frequently incompletely mixed with the pulp slurry, causes corrosion as well as the channeling in the chlorination tower. Secondly, when the chlorine is not evenly mixed with the pulp slurry, the brightness of the resulting paper is not even. Further, a longer reaction time is required to complete the oxidation of the lignin. Finally, a higher chlorine concentration is required to establish the proper brightness of the paper, and therefore, more chlorine is required.
The next stage in development of chlorinators included the use of a single stage completely mixed system. This consists of a tank with an agitator that completely mixes the pulp slurry and chlorine before it is delivered to the chlorination tower. To a large extent, this system solves the problem of uneven distribution of the chlorine. It also blends out any fluctuations between the concentration of the pulp and the chlorine. Although the concentration of chlorine can be carefully controlled, the amount of fiber being introduced into the system varies since the consistency of the pulp slurry fluctuates. The single stage system does introduce a problem of under and over bleaching of fibers. This is due to the variation in the residence time of the fibers in the system. Since a fiber entering the system may go directly out of the system, there is a probability that certain fibers are under bleached. On the other hand, fibers may remain in the system for a very long period and are thereby over bleached.
The next improvement came with the introduction of chlorinators being multi-stage systems. Here the chlorine and pulp are mixed in a first stage and then passed into at least one additional stage wherein the mixture is agitated and finally passed into a chlorination tower. The multi-stage systems are able to solve the retention problems and thereby inhibit under and over bleaching of the fibers. Just as there is given probability that a fiber could pass through a single stage in a very short period of time, there is an equal probability that the same fiber may take an extremely long period of time to pass through a second stage. Therefore, by increasing the number of stages through which the fiber must pass, the probability of a proper retention time increases. Another advantage of the multi-stage system is that by mixing the mixture in several stages, there is less chance for wet chlorine to pass into the tower.
Another distinct advantage of the multi-stage system is related to the reaction between chlorine and the pump being a first order reaction. If the concentration of the chlorine in the mixture decreases by 50 percent, the total reaction time increases by approximately 50 percent. In the multi-stage system, each stage has a decreasing concentration of chlorine because the chlorine is continually being used up in oxidizing the lignin. This results in the reaction proceeding at a faster rate in the earlier stages, as compared to a single stage system, since these earlier stages have a higher concentration than is found in a completely mixed single stage system.
One of the advantages of increasing the reaction rate is that the chlorination tower need not be quite as large to permit the reaction to be completed. It has also been found that agitating the solution increases the reaction rate and decreases the amount of chlorine required for bleaching. Thus, by increasing the number of stages, mixing increases, the reaction rate increases, and the amount of chlorine required decreases.
It is an object of the present invention to reduce the total reaction time between chlorine and pulp.
It is a further object of the present invention to reduce the amount of chlorine required in the reaction between chlorine and pulp.
It is a further object of the present invention to change the power level between the stages of a multi-stage chlorinator.
It is a further object of the present invention to reduce the size of the equipment used in the oxidation of pulp.
It is a further object of the present invention to reduce the power consumed in oxidizing pulp.